Cable and telephone services have traditionally been offered to subscribers by different companies. The telephone services developed around a unified national network that utilized twisted wire pairs to connect the subscriber's telephone to a central office. With the appearance of local cable television services, new networks of coaxial cables were introduced locally to carry the radio frequency video signals to the subscriber's televisions, video cassette recorders, set top boxes and the like. As a result, having two sets of wires coming into their buildings is common for subscribers, a twisted wire pair for telephone services, and a coaxial cable for video services.
In recent years, the division between telephone and cable services (sometimes referred to as "broadband") has blurred. Cable companies have started offering telephone and Internet access services using the wide bandwidth made available by their coaxial cable networks. Telephone companies, in turn, have started offering other services, including television services and high speed digital network services. Telephone companies have become more numerous giving subscribers choices among several companies instead of just one regional Bell company. This expansion of services often requires subscribers to have several sets of wiring installed in their buildings. Where cosmetic appearances are important, installing new wires in an existing structure can be expensive. One solution to the problem is to merge the telephone signals and the broadband signals onto one set of wires outside the buildings, route them through the existing wires within the building, and then separate them at their destination inside. For existing buildings this approach avoids the need to lay new wiring. In new structures this approach saves on the total amount of wiring that needs to be installed.
A merged telephone/broadband solution must deal with the different requirements of balanced telephone signals (including the power, voice, ringing, and other signals) and unbalanced broadband signals used for video and high speed data. Twisted wire pairs are suitable for low frequency balanced signals over long distances because the physical placement of each wire in the pair results in equal exposure to external electromagnetic noise. When external noise is induced on a twisted wire pair cable it generally appears on each wire of the pair simultaneously. The equipment receiving the induced noise on the twisted wire pair can filter it out by canceling the noise signal on one wire with the equal noise signal on the other wire. Twisted wire pair, however, is not well suited for the higher radio frequencies of cable television because the wires act as antennas that broadcast the broadband signals out into space.
In coaxial cable design, an outer conductor surrounds and shields the inner conductor used to carry the information. The outer conductor prevents the radio frequency signals on the inner conductor from broadcasting outside the cable. This helps make coaxial cable suitable for carrying broadband signals over long distances. The outer conductor also acts to shield the inner conductor from external electromagnetic noise sources. Any external noise reaching the coaxial cable is captured by the outer conductor, none of the noise is induced on the information carried by the inner conductor. To dispose of the unwanted noise signals, the outer conductor is normally connected to earth ground at regular intervals.
While coaxial cable works well for the unbalanced signals of cable television, it is not ideal for the balanced telephone signals. Carrying telephone signals on coaxial cable causes external noise to be induced in only one of the two telephone lines--the line carried on the outer conductor. This results in an imbalanced noise on the telephone lines that cannot be distinguished from the voice information. Another problem with telephone signals on coaxial cable is that the telephone signals include electrical power. For a coaxial cable to carry this power, the outer conductor would have to be isolated from earth ground so as not to short the electrical power to ground.
U.S. Pat. No. 5,440,335 issued to Beveridge on Aug. 8, 1995 provides an approach to carrying merged telephone and broadband signals, over long distances using a pair of coaxial cables. By carrying the telephone's tip line on the inner conductor of one coaxial cable and the telephone's ring line on the inner conductor of the second coaxial cable, the balance between the two telephone lines is maintained. The patent also discloses using the inner conductor of each coaxial cable to carry a broadband signal frequency multiplexed above the telephone signals. An advantage of this approach is the ability to supply two independent broadband channels and one telephone service to a subscriber using only two coaxial cables. A limitation of this approach is the need to route two sets of coaxial cables to each subscriber.
A practical solution for combining telephone and broadband signals on a signal cable is to use one coaxial cable for only short distances in electromagnetically quiet areas. For example, in existing multiple dwelling apartment buildings all of the coaxial cables used for cable television are routed from a central lock box, usually located just outside the building, through the walls of the building to the individual apartments. By inserting the telephone signals onto the coaxial cables in the lock box, and then returning the telephone signals to twisted wire pairs once inside the apartments, the noise levels induced on the telephone signals can be kept to acceptably low levels. An advantage in this approach is that new telephone services can be provided to cable television subscribers without the expense of installing new wire s throughout the building.